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Fig. 2 Malignant transformation of a single cell triggering carcinogenesis in vivo. (A) At 1 day post-fertilization (dpf), a single cell in a zebrafish brain was photoinduced to express the oncogene KRASG12V, identified (white arrowhead) within ~1 hr by the blue fluorescent H2B-mTFP. Membrane-bound mRFP is used as tracer. Transient (24 hr) dexamethasone (DEX) activation of Ventx is done following photoactivation. The probability p of inducing one or more (blue fluorescent) cells is shown on the right panel, together with the Poisson distribution (red curve) expected for the independent induction of k cells (error bars are statistical errors on the mean: ?=p(1?p)/N??????????? , where N is the total number of observed embryos). (B) At 1 day post-induction (1 dpi), the activated cell may have divided (middle panel) giving rise to two mTFP+ (blue fluorescent) cells (white arrowhead) or may not have divided (left panel). The probability of observing k blue fluorescent cells at 1 dpi is shown on the right panel. (C) At 3 dpi the original cell expanded clonally (white arrowheads) by short-range dispersal within the brain. At 4 dpi the brain has been colonized (middle panel) by the progeny of the activated cell that display tumor growth as well as dispersal in the head or entering into the cardiovascular system (red arrowhead). At 5 dpi (right panel), a tumor mass is formed. (D) Confocal microscopy of a larval head displaying tumors (white arrowheads) and dispersal in the trunk (red arrowheads). (E) Histopathological sections of larval brain (dorsal view). Dashed lines (1?5) indicate sections shown in (F). Hematoxylin and eosin (H&E) staining of brain at 5 dpi of KR+VX-induced larva (depigmented) is compared to normal brain (Ctrl, melanocytes in brown). At 5 dpi, the optic tectum (red arrows), the tegmentum (black arrow), and hypothalamus (red arrowhead) are infiltrated by a dysplastic tumor, progeny of the initial induced single cell. An asterisk (*) indicates the eye and a white arrow the otic vesicle. Scale bars and the body axes (a: anterior; p: posterior; d: dorsal; v: ventral) are shown. T=telencephalon; M=mesencephalon; C=cerebellum; Mc =melanocytes (yellow arrows).